Conventionally, as a semiconductor photodetector used for optical communication which is effective over a range of wavelengths of 1 through 1.6 .mu.m, there has been known a PIN photodiode (IEEE Electron Device Letters, pages 283 through 285, Vol. EDL-2, 1981) or an avalanche photodiode (IEEE Electron Device Letters, pages 257 through 258, Vol. EDL-7, 1986), in which an InGaAs layer (for example, In.sub.0.53 Ga.sub.0.47 As layer) disposed on an InP substrate so as to align in lattices is served as a light absorptive layer. In particular, the latter has been in practical use for long-distance communication because it can exhibit an internal gain effect and a high speed response resulting from the avalanche multiplier effect.
FIG. 1 illustrates a cross-sectional view of a typical InGaAs avalanche photodiode. An avalanche photodiode will be hereinafter referred to as APD. Formed on an n-type InP substrate 1 are an n-type InP buffer layer 2, an n-type InGaAs light absorptive layer 3, an n-type InP avalanche multiplier layer 4, an n-type InP cap layer 5, a p-type light receptor area 6, a p-type guard ring region 7 and a passivation film 8. In addition, a p-side electrode 9 is connected to the light receptor area 6 while an n-side electrode 10 is connected to the substrate 1.
According to this APD operating principle, among the photo-carriers generated at the InGaAs light absorptive layer 3, holes are injected into the InP avalanche multiplier layer 4. Since a high voltage is applied to the InP avalanche multiplier layer 4, an ionizing collision takes place there resulting in a multiplying characteristic. In this case, it is known that the noise and response characteristics, which are important from the point of the element characteristics, are governed by the random ionizing process of the carriers in the process of multiplication. To be concrete, the greater the difference between the ionization factor of the electron and that of the hole at the InP layer, which is the multiplier layer, is, the greater the ratio therebetween can be taken (assuming that the ionization factors of the electron and the hole be each .alpha. and .beta., if .alpha./.beta.&gt;1 then the electron corresponds to the main carrier which causes the ionizing collision, and if otherwise, then the hole serves as the main carrier for carrying out the ionizing collision), which is desirable from the point of the element characteristics.
However, the ratio between the ionization factors (.alpha./.beta. or .beta./.alpha.) is determined in terms of the physical property of the material used, and is equal to .beta./.alpha.=2, at most, for InP. This is remarkably small as compared with the .alpha./.beta.=20 for Si having a low noise characteristic. Therefore, in order to realize a further lower noise characteristic and a higher response characteristic, some epoch-making material technology is called for.
In connection with this, F. Capasso et al proposed a superlattice APD which aims at achieving the high sensitivity and a wide range of bandwidth through the increase of the ratio .alpha./.beta. by utilizing the discontinuity (discontinuity value .DELTA.Ec) of the energies at the lower end of the conduction band to promote the ionization of the electron. The same example is described in Applied Physics Letters (Pages 38 through 40, Vol. 40, 1982). On the other hand, not in connection with APD, it is also known that the band structure can be changed by applying a stress to the superlattice structure of the semiconductor and, in particular, the degeneration of the energy level of the heavy and light holes is released in the valence band. The same example is described in Journal of Applied Physics (pages 344 through 352, Vol. 67, 1990).
As described above, in the superlattice APD, .DELTA.EC greatly lends itself to the improvement of the ratio between the ionization factors. However, in the superlattice APD, at the same time, the holes are piled up, and the bandwidth is suppressed due to the discontinuity (discontinuity value .DELTA. Ev) of the energies at the upper end of the valence band.